High frequency receivers for satellite or ground terminal applications require low phase noise oscillators. As the utilized spectrum moves into the millimeter (mm) wavelength bands, phase noise becomes a greater concern for communication systems and particularly digital satellite communication systems. Crystal stabilized oscillators are devices that are suitable up to frequencies near 1 gigahertz (GHz). This is because the highest fundamental frequency achievable from a crystal is a few hundred megahertz (MHZ), and phase noise scales as N.sup.2, where N is the frequency multiplication factor. At higher frequencies, dielectric-resonator-oscillators (DROs) are the component of choice. Despite their high Q values (low phase noise), these components are inadequate to prevent bit error rate (BER) degradation and are expensive because it cannot be produced lithographically.
U.S. Pat. No. 4,887,052 to Murakami, et al. describes a tunable oscillator utilizing a thin film ferromagnetic resonator which is phase locked and operates in the GHz range for satellite communications. The Murakami, et al. device employs a phase locked loop to provide a reference signal to which the ferromagnetic resonator can lock on. Thus, this approach necessitates a separate reference signal source. Furthermore, the use of thin ferromagnetic film technology requires rather complicated circuitry to supply a control current and high magnetic field.
U.S. Pat. Nos. 5,589,845 and 5,472,935 to Yandrofski, et al. describe the application of ferroelectric and superconducting thin films to a variety of tunable microwave components.
U.S. Pat. Nos. 5,617,104 and 5,496,796 issued to Das describe a high temperature superconducting (HTS) tunable ferroelectric transmitting system which is based in a bulk single crystal requiring the application of kilovolts to induce tuning. Das describes several different realizations of his systems which include an antenna, a filter and an oscillator that require a microprocessor. This configuration would appear to make the system more cumbersome and expensive, particularly for integration into miniaturized systems and for high volume production.
U.S. Pat. No. 4,873,496 to Ogihara, et al. discloses a tunable local oscillator based in a common gate serial feedback type. The Ogihara, et al. device employs an Yttrium-Iridium-Garnet (YIG) ferrite component as the resonant element and a GaAs FET as the active component. Although the Ogihara, et al. approach capitalizes on the utilization of the second harmonic rather than the fundamental so as to reduce the magnitude of the external dc magnetic field required for tuning, the circuit implementation appears to be rather difficult because of the use of a magnetic coil and a current source to generate the magnetic field. This increases the size and complexity of the circuit rendering it less compatible for integration in monolithic microwave integrated circuit (MMIC) based working systems.
U.S. Pat. No. 5,289,139 to Fiedziuzsko, et al. describes a so-called "push-push" oscillator circuit based on an annular resonator as a stabilizing element of a two branch local oscillator. There appears to be no mechanism in this patent which allows for adjusting the operation frequency of the oscillator. Also, the oscillator's only feature for phase noise reduction is the potentially high quality factor (Q) of the superconducting resonator (since phase noise is inversely proportional to the square of Q).
U.S. Pat. No. 4,097,826 to Knox, et al. describes several versions of insular wave guides, dielectric-based ring resonator filters. These filters are intended for broad frequency range operation (i.e., 1 to 1,000 GHz), and generally could have applications for communication subsystems. In particular, this patent appears to be concerned with the filter component of the receiver rather than with the local oscillator component. The local oscillator described therein is tuned by either a tuning short, by varying the bias current applied through the coaxial transmission line, or by applying dc voltages to a diode forming part of the oscillators as seen in FIGS. 2 through 6.
U.S. Pat. No. 4,555,683 to Sorger, et al. describes different versions of tunable resonators and filters implemented with a disk shaped ferrimagnetic disk. This patent relates to tunable resonators and components where the tuning element is a ferrimagnetic single crystal rather than a ferroelectric thin film. Tuning is induced by an externally applied magnetic field generated by current transmission lines wrapped around the ferrimagnetic disk.
U.S. Pat. No. 5,059,927 to Cohen describes a lumped circuit, ultra-wide band tuning voltage controlled oscillator (VCO) with a Gunn diode as an active element which exhibits a 20 GHz tuning range at V-band. This device reduces the phase noise of the oscillator by enhancing the Q of the oscillator, and provides full microwave band coverage with fewer VCOs, because of its ultra-wide band range, smaller size and lower cost than most commercially available VCOs. The tuning element in this device is a diode whether it is a Gunn or Avalanche diode.
U.S. Pat. No. 4,945,324 to Murakami, et al. describes a tunable filter where the tunable element is comprised of ferromagnetic thin films (Yttrium Iron Garnet, "YIG") hosted in a non-magnetic substrate (Gallium Gadolinium Garnet, "GGG"). This patent addresses typical problems associated with ferromagnetic-based tunable resonators such as limited tunable frequency band, variation of the three dB bandwidth across the tunable frequency band and diminishes spurious characteristics by controlling or adjusting the coupling coefficient across the band. In particular, the patent appears to deal with a tunable filter with the tunability being attained through the use of ferromagnetic films.
U.S. Pat. No. 4,853,660 to Schloemann, et al. describes a multi layer ferromagnetic circuit capable of being tuned with an appropriate magnetic field. Again, this patent is representative of tunable technology enabled with the use of ferromagnetic materials as tunable elements as opposed to ferroelectric materials. Tunability is produced by the application of an external dc magnetic field which intrinsically makes the circuit implementation more difficult.
U.S. Pat. No. 5,334,958 to Babbitt, et al. has similarities to U.S. Pat. No. 5,561,407 to Koscica, et al. in that they describe a phase shifter that employs a slab of ferroelectric material upon which a microstrip line is patterned. This same line is then biased and an electric field is generated in the slab perpendicular to the propagation velocity. This is a form of a ferroelectric phase shifter.
Other patents of interest include U.S. Pat. No. 5,382,959 to Pett, et al. which relates to a high performance circularly polarized antenna complex. U.S. Pat. No. 5,210,541 to Hall, et al. also involves antenna applications. U.S. Pat. No. 5,434,581 to Raguenet, et al. describes a technique for enhancing the bandwidth of a microstrip patch antenna or an array of such patches. U.S. Pat. No. 5,124,713 to Mayes, et al. describes a particular type of antenna element. U.S. Pat. No. 5,086,304 to Collins relates to a cost competitive alternative to reflector type antennas.
There still exists a need for a tunable local oscillator which offers a high Q value (low phase noise) with a locked mode to further reduce phase noise. Such a tunable local oscillator would operate at frequencies at least as high as 60 GHz and preferably with approximately a 5% tuning range around the carrier frequency. The frequency tuning would be based on altering the dielectric constant of the ferroelectric film by applying a fixed dc voltage. The tunable local oscillator would have the ability to be implemented to perform at both room temperature using conventional conductors and at cryogenic temperatures using superconductors. Preferably, a portion of the output signal at the fundamental frequency would be sampled with a diplexer feedback circuit. An error signal would be used to tune a composite ferroelectric/superconductor resonator thereby keeping it locked to a predetermined frequency. Wideband tuning would be achieved by simultaneously controlling the crossover frequency of the diplexer and the fundamental frequency of the resonator. Such a tunable local oscillator would include a technique for capitalizing on the high Q of superconductors and the frequency agility of ferroelectric thin films simultaneously. It would allow a method for dynamically tuning the circuit. The tunable local oscillator would alleviate bit error rate (BER) degradation for various applications and in particular digital satellite communication systems.